Iron-catalysed alkenylzincation of allenes via electrophilicity reversal

Abstract

Given the structural characteristics of allenes, nucleophilic additions usually occur at the electron-deficient central carbon atom of allene. Here we report an iron-catalysed alkenylzincation reaction of terminal allenes that shows abnormal regioselectivity, wherein the electrophilic zinc moiety from an organozinc reagent is incorporated at the electron-deficient central carbon atom of the allene. This alkenylzincation reaction shows broad functional group compatibility and excellent regio- and stereoselectivities. Using this method, we accessed cis-1,4-dienylzinc reagents and their corresponding polysubstituted 1,4-diene derivatives, which are notoriously challenging to prepare via conventional routes. Mechanistic studies revealed that an unexpected reversal of the electrophilicity of the allene carbons is realized through the electron donation from the Fe(0) to the allene via π back-bonding, resulting in the observed abnormal regioselectivity.